Field of the Invention
The present invention relates to biodegradable base oils for lubricants and functional fluids comprising polymethylalkanes having terminal methyl groups and methylene and ethylidene groups. These polymethylalkanes are useful as base oils in formulations with conventional additives and mixtures of other biodegradable or non-degradable base oils.
Discussion of the Background
Owing to the increased environmental awareness and the more stringent environmental legislation, lubricating oils and functional fluids, such as hydraulic oils, based on non-biodegradable base oils are being examined more and more critically since they can enter the environment, especially the soil, as a result of leaks after accidents or even during the replacement of these fluids. Soil contaminated in this manner must be cleaned at considerable expense. It is therefore increasingly desirable to find biodegradable lubricating oils and functional fluids.
Three product classes have been identified which are useful as base oils for biodegradable lubricants and functional fluids: synthetic esters, natural esters and polyalkylene glycols (PAG), mainly polyethylene glycols (PEG). These classes of compounds exhibit good biodegradability as well as very good lubricating properties, such as, for example, improved viscosity/temperature behaviour, high lubricating capacity and heat resistance.
In addition to these positive properties, however, these base oil types have some disadvantages which greatly limit their use in certain areas. The biodegradable PAGs are, as a rule, water-soluble. Thus, it is difficult to detect leaks, and contamination is rapidly distributed. Furthermore, PAGs frequently have high fish toxicity. Further, PAGs are largely incompatible with the conventional coatings and sealing materials.
Moreover, the conversion of plants which produced fluids based on non-biodegradable base oils into plants producing fluids based on the biodegradable base oils can be expensive. These plants have to be completely emptied and cleaned, since PAGs are immiscible with products based on mineral oil.
Synthetic as well as natural esters have only low hydrolytic stability. Their compatibility with sealing materials is also problematic, as in the case of the PAG. Furthermore, special additive packages are required for the formulation of ready-to-use products.
Natural esters, such as, for example, rapeseed oil, also have the disadvantage of low oxidation stability, exhibit poor low-temperature behaviour in some cases and have fluctuating product quality.
Hydrocarbons based on mineral oils and synthetic hydrocarbons, for example poly-.alpha.-olefins (PAO), do not have the above-mentioned disadvantages but are only slowly biodegradable since they are mixtures of straight-chain, cyclic and/or highly branched compounds. Where there is a high proportion of the readily degradable n-alkanes, such mixtures are in the form of waxes at room temperature and are unsuitable as lubricants.
Only dimeric .alpha.-olefins (PAO 2), which are biodegradable to an extent of 45 to 92%, according to CEC L-33-T-82, constitute an exception (see for example, EP-A 0 468 109). However, owing to their low viscosity (KV=2 mm.sup.2 /s at 100.degree. C.), these products as such can be used as base oils only in a few exceptional cases and are added to lubricating oil formulations mainly in low concentration. On the other hand, trimeric .alpha.-olefins (PAO 4, KV=4 mm.sup.2 /s at 100.degree. C.) exhibit a substantially lower biodegradability of only 24% in the above-mentioned CEC test. These components are therefore unsuitable as base oils for biodegradable functional fluids and lubricants.